Currently, Orthogonal Frequency Division Multiplexing (OFDM) has been widely used in transmission systems of various digital communications such as digital terrestrial broadcasting and IEEE802.11a. OFDM, in which a plurality of narrowband digital modulation signals are frequency-multiplexed using multiple orthogonal sub-carriers, is an excellent transmission system that efficiently utilizes frequencies. In addition, in OFDM, each symbol period consists a cycle of a valid symbol period and a guard interval. Accordingly, signals in the valid symbol period are partially copied into the guard interval, which can reduce the impact of the inter-symbol interference caused by multipath interference. Thus, OFDM is robust against multipath interference.
However, in OFDM, since one symbol length in a narrowband digital modulation signal is longer than that in a broadband digital modulation signal, OFDM is sensitive to a time variance in a channel fading environment that occurred during the mobile reception and the like. Moreover, in the channel fading environment, in addition to a time variance in an amplitude of a reception signal that occurs as a result of delay dispersion due to multipath interference, a frequency variance called Doppler shift occurs. This Doppler shift destroys the orthogonality among sub-carriers, and thereby causing mutual interference among the sub-carriers. Consequently, it is difficult to perform correct demodulation. This interference occurred among the sub-carriers is called inter-carrier interference (ICI). To suppress the degradation of the communication quality caused by the ICI is the key to improve the transmission characteristic.
In recent years, several approaches have been made to improve this deterioration caused by the ICI. One of the approaches is disclosed in Nonpatent Document 3.
FIG. 40 is a block diagram showing the configuration of an ICI removal unit described in Nonpatent Document 3.
A channel characteristic estimation unit 4001 estimates a channel characteristic from a signal obtained by FFT (indicated as Y). A tentative equalization unit 4002 divides the post-FFT signal by the channel characteristic to estimate a transmission signal (indicated as X˜(s), where “s” represents the current symbol number.)
Based on the estimated channel characteristic, a channel characteristic linear differential calculation unit 4003 calculates, using Expression 1, a linear differential (indicated as H′(s)) of a channel characteristic H(s) by calculating a difference of the channel characteristics between preceding and subsequent symbols of the current symbol, and outputs the linear differential H′(s) to a multiplication unit 4004.H′(s)=(H(s+1)−H(s−1))/(2·Ts)  (Expression 1)
In Expression 1, the letter “Ts” represents an OFDM symbol length.
Subsequently, the multiplication unit 4004 estimates an ICI component K(s) with use of Expression 2 by calculating the tentatively equalized signal X˜(s), the linear differential H′(s), and a constant matrix Ξ.K(s)=Ξdiag(H′(s))X˜(s)  (Expression 2)
The letter, “Ξ” is as expressed in Expressions 3 and 4.
                    Ξ        =                  (                                                    0                                                              ζ                  1                                                            …                                                              ζ                                      N                    -                    1                                                                                                                        ζ                                      -                    1                                                                              0                                            …                                                              ζ                                      N                    -                    2                                                                                                      ⋮                                            ⋮                                            ⋱                                            ⋮                                                                                      ζ                                      1                    -                    N                                                                                                ζ                                      2                    -                    N                                                                              …                                            0                                              )                                    (                  Expression          ⁢                                          ⁢          3                )                                          ζ          n                =                              -                          1              2                                -                      j                          2              ⁢                                                          ⁢                              tan                ⁡                                  (                                      π                    ⁢                                                                                  ⁢                                          n                      /                      N                                                        )                                                                                        (                  Expression          ⁢                                          ⁢          4                )            
In this description, diag (An(s)) is expressed as a square matrix having N lines×N columns as shown below (n=0, . . . , N−1). Note that the letter “n” is a carrier number and the letter “N” is the total carrier number.
      diag    ⁢                  ⁢          (                        A          n                ⁡                  (          s          )                    )        =      (                                                      A              0                        ⁡                          (              s              )                                                0                          …                          0                                      0                                                    A              1                        ⁡                          (              s              )                                                …                          0                                      ⋮                          ⋮                          ⋱                          ⋮                                      0                          0                          …                                                    A                              N                -                1                                      ⁡                          (              s              )                                            )  
Subsequently, a subtraction unit 1005 performs subtraction to remove, from the post-FFT signal Y, the ICI component estimated from Expression 2.
Nonpatent Document 1: ARIB STD-B31
Nonpatent Document 2: IEEE Std 802.11a-1999
Nonpatent Document 3: Karsten Schmidt et al, “Low Complexity Inter-Carrier Interference Compensation for Mobile Reception of DVB-H,” 9th International OFDM-Workshop 2004, Dresden (Page 72-76, FIG. 4)
Patent Document 1: JP Patent Publication No. 2004-519900